At present, LEDs are mainly used as backlights of liquid crystal displays (LCDs) used for a mobile phone, a personal digital assistant (PDA), a notebook computer, and the like. However, because efficiency and luminance have been significantly improved with the development of LED manufacturing technology, LEDs are also being widely used as general lighting, a guard lamp, a road lamp, and the like as well as a light source of a large LCD such as a television (TV). Fluorescent lamps currently in wide use are expected to be replaced with LEDs within 10 years due to the long life, environmental friendliness, and continuous optical efficiency improvement efforts of LEDs.
In general, when an LED is used as general lighting in a constant current source driving type, a commercial power-supply voltage of alternating current (AC) 220 V should be subjected to AC-direct current (DC) conversion. However, when a simplest diode rectifier is used without using a transformer, a DC voltage after the AC-DC conversion becomes about 310 V, which is a very high voltage to directly drive the LED. As a result, a voltage obtained by converting a primarily rectified voltage into a voltage suitable to drive the LED using a step-down DC-DC converter is used.
An LED current control circuit in a representative step-down DC-DC converter type is illustrated in FIG. 1. Referring to FIG. 1, a current flowing through the LED is detected using a resistor Rs in a source or emitter of a switching element SW. Although the current flowing through the LED may be detected through a voltage across the resistor Rs when the switching element SW is turned on, the current flowing through the LED is not detected if the switching element SW is turned off. Accordingly, only a maximum value of an output current is controlled by detecting and controlling the current only when the switching element SW is turned on. In this case, there is a problem in that an average value of the current flowing through the LED is not actually controlled because of a change according to magnitude of an input voltage and magnitude of a voltage across the LED.
To solve this problem, a method of detecting the current flowing through the LED using a separate current detection circuit including an insulated current sensor as illustrated in FIG. 2 may be used. However, there is a problem in that a simple current sensor is not available because high-voltage insulation characteristics of a current detection circuit are required in a high-voltage environment and a size and cost of the entire system are increased because a separate sensor having high-voltage insulation characteristics should be added.